It is important to have some idea of the count rate expected from each
target, both for reference during the mission, and for instrument
safety considerations. The program hutsim, described earlier in
this manual, provides a mechanism for simulating spectra of each object and
estimating the count rates expected, including airglow lines.
(The plots from these simulations can also be placed in
the Target Book used by the payload specialists to set up an observation
and verify that all is proceeding nominally.)

Sources whose predicted count rates exceed 5000
counts s-1 exceed safety limits for the detector. High count rates can also present
dead time problems as described earlier in this manual.
These situations can be handled
in a number of ways. For extended sources, selecting a smaller
aperture may resolve the problem. For point sources, however, one may
have to choose something other than the full telescope aperture. The
flux may be attenuated by a factor of two by closing one or the other
of the ``half aperture" doors at the front of the telescope. If still
too bright, the large aperture doors can be closed, and one of two
small aperture door positions (50 cm2 or 1 cm2) can be utilized. All of
these situations can be simulated using hutsim in order to both derive
expected count rates, and analyze the quality of the data resulting
from one of these alternate door states.